This application claims the priority of German patent document 199 13 746.3, filed Mar. 26, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for detecting exhaust-gas-impairing and catalyst-damaging misfires in internal-combustion engineS.
Governments increasingly demand monitoring of an engine to detect combustion misfires in the course of an on-board diagnosis; and the methods of this type are generally known. For example, German Patent Document DE 40 02 208 A1 describes a method for detecting combustion misfires in an engine based on a lambda signal generated by a lambda probe. In this method, the amplitude of a signal fraction which has the frequency of the camshaft is determined, and the average of successive amplitude values is calculated. If the average amplitude value exceeds a threshold value, a conclusion is drawn that misfires are present.
International Patent Document WO 92/11522 also describes a diagnostic device for detecting combustion misfires in an internal-combustion engine. In this publication, it is generally pointed out that three groups of detection methods are known. The first group considers process output values, such as erratic running, noise, vibrations or the lambda probe signal. The second group of diagnostic methods considers process input values, such as the ignition current or the output signals of the end phases for the injection valves. The third group finally considers the process itself by means of a pressure determination in the combustion space.
However, there are different types of combustion misfires, and a distinction is generally drawn between exhaust-gas-impairing and catalyst-damaging misfires.
One objective for the monitoring of exhaust-gas-relevant misfires is to report a fault when a defined exhaust gas threshold value is exceeded. For example, a misfire rate can be determined for four respective intervals at 1,000 r.p.m.
In the case of still higher misfire rates, however, the catalyst may be damaged when unburnt fuel flows through it and it is overheated. In this circumstance, a fault must be reported and the concerned cylinder or cylinders must be switched off. According to the operating range of the engine, the analysis of catalyst-damaging misfires takes place, for example, in one or several intervals at 200 engine revolutions.
The disadvantage of the above-mentioned known diagnostic techniques for detecting combustion misfires is that all misfires are accorded the same treatment, no matter what the causes are. That is, no differentiation is made as to whether the fuel supply or the ignition is interrupted, or whether another fault exists. Consequently, for reasons of safety, in the event of an interruption of the fuel supply, it is treated (incorrectly) as a catalyst-damaging combustion misfire, even though damage to the catalyst is not occurring at all or will occur only at a much higher misfire rate. A fault report and a cylinder switch-off therefore takes place prematurely, at a time when it would not be necessary.
German Patent Document DE 40 19 573 A1 discloses a method for detecting combustion misfires and/or a non-uniform injection in an internal-combustion engine, by analyzing the fraction or the partial pressure of the oxygen in the exhaust gas to determine the type of combustion misfire.
In the publication xe2x80x9cMethods of On-Board Misfire Detectionxe2x80x9d, SAE Paper 900232, 1990, by G. Plapp, M. Klenk and W. Moser, discloses an arrangement for combustion misfire detection, either by analysis of the lambda probe signal or of the erratic running of the engine. By analyzing the lambda probe signal, it can be determined whether the combustion misfire was caused by a defective injection or by a defective ignition.
However, the above-mentioned techniques for detecting combustion irregularities by analysis of the lambda probe signal are relatively sluggish and partially inaccurate.
It is therefore an object of the present invention to provide method of the initially mentioned type by which can accurately detect combustion irregularities and can easily be determine the type of misfires.
This and other objects and advantages are achieved by the method according to the invention, in which the occurrence of a combustion misfire is recognized by detecting an erratic rotational speed or erratic running of the engine. On the other hand, the type of misfire is determined by analysis of the lambda probe voltage. In this case, the lambda probe voltage is determined with regard to the recognized combustion misfire, and as a function of this probe voltage, a conclusion will then be drawn with respect to the type of misfire.
To determine whether unburnt fuel reaches the catalyst, the lambda probe arranged downstream of the catalyst is used. When the ignition fails, the probe behind the catalyst will detect a xe2x80x9crich mixturexe2x80x9d, indicated by a voltage above a defined limit value. When the injection fails, the catalyst is not acted upon by unburnt fuel so that the probe signal indicates a xe2x80x9clean mixturexe2x80x9d, in which case the voltage is lower than a defined limit value.
On the whole, the probe voltage is therefore used as a decision criterion for indicating the type of misfire and is therefore included in a two-stage method. In the first stage, the combustion irregularity is determined by analysis of the rotational speed or another detection of an erratic running.
According to an advantageous embodiment of the invention, the probe signal can be filtered for analysis. As an alternative, the signal average can also be determined in an interval during which misfires occur. In the latter case, preferably the gas running times and/or the times for the flooding of the catalyst with oxygen can also be taken into account.
In a preferred embodiment of the invention, it is determined whether the lambda probe voltage exceeds a first limit value. As a function of the lambda probe voltage, a conclusion is then drawn with respect to the type of misfire: that is, a faulty ignition, a faulty injection or another defect.
In an alternative embodiment, two limit values are determined. If the probe voltage is above the upper limit value, a faulty ignition is assumed. If the probe voltage is below the lower limit value, a faulty injection is assumed.
A transition probe or a broad-band probe can be used as the lambda probe.
Because a defectively operating cylinder is switched off at a designated rate of catalyst-damaging misfires, switch-off can take place precisely when the catalyst would actually be damaged. A fault report with respect to xe2x80x9ccatalyst-damaging misfiresxe2x80x9d would therefore take place later in comparison to conventional diagnostic operations. This is permissible because damage to the catalyst does not in fact occur in the case of an exhaust-gas-impairing misfire. On the whole, a switch-off of a cylinder can therefore be delayed or prevented.
By storing the misfires and the type of misfires in a memory (particularly in the memory of a diagnostic unit), the search for defects in the repair shop can be considerably simplified.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.